The invention resides in an anode substrate with a bi-polar plate disposed thereon for a high temperature fuel cell which anode substrate has a phase in which it does not act as catalyst with regard to methane-vapor reforming reactions and a phase in which it acts as a catalyst.
A high temperature fuel cell (SOFC) comprises a stack of fuel cells with an associated periphery. The basic components in a fuel cell are the electrolyte [material for example yt-trium-stabilized zirconium dioxide (YSZ) ; ZrO.sub.2 -8mol%Y.sub.2 O.sub.3 ], the cathode[material for example strontium-doped lanthanium manganese oxide (LSM); La.sub.1-x-y Sr.sub.x MnO.sub.3-2 ] and the anode consisting of ceramic (=non catalytically acting) as well as as a metallic (=catalytically acting) phase [for example nickel-YSZ Cermet; 40 vol% Ni/ZrO.sub.2 -8 mol% Y.sub.2 O.sub.3 ]. For a series and/or parallel arrangement of several cells in a stack a fourth arrangement of several cells in a stack a fourth component is required; a bi-polar plate (material for example earth alkaline metal-doped lanthanium chromate; La.sub.x Me.sub.x CrO.sub.3 (Me=Sr, Ca) or a high temperature Cr-alloy; Cr.sub.5 Fe.sub.1 (Y.sub.2 O.sub.3)] [A Hammou, Adv. Electrochem. Sci. and Engin. 2(1992) 87-139; N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563-88; D. Stolten and W. Schafer, in ; Technische Keramische Werkstoffe, J. Kriegsmann (editor), Deutscher Wirtschaftdienst, John Freyland GmbH, Cologne, 1994, Chapter 8.5.2.0]
A stack of fuel cells is set up in accordance with two concepts. The first concept is based on tubes which have support functions and which must provide for the mechanical stability of the cell. The tubes consist either of an inert material or of one of the two electrode materials. The functional components are disposed on these tubes as layers arranged in a specific geometry and order. The second concept utilizes self-supporting electrolyte foils and bi-polar plates in a flat geometry. The two electrodes are disposed as layers on the electrolyte foil. The electrolyte foil as well as the bi-polar plate must insure the mechanical stability of the arrangement. [A Hammou, Adv. Electrochem. Sci. and Engin. 2(1992) 87-139; N.Qminh, J. Am. Ceram. Soc. 76 (1993) 563-88; D. Stolten and W. Schafer, in ; Technische Keramische Werkstoffe, J. Kriegsmann (editor), Deutscher Wirtschaftdienst, John Freyland GmbH, Cologne, 1994, Chapter 8.5.2.0]
Both stacking concepts mentioned above have their specific disadvantages. Because of the geometry, the tube concept provides for relatively long charge transport paths which results in relatively high ohmic losses. Because of a relatively large-pore porosity of the porous support tube a substantial minimum thickness is required for the electrolytic layer in order to make it gas-tight. The flat cell concept has the disadvantage that the self supporting electrolyte foils have to have a certain minimum thickness in order to insure mechanical stability. This also limits a self-supporting electrolyte foil to a certain size (effective surface area). As a result, in both concepts, the stacks can be operated without substantial losses only above 900.degree. C. because of the relatively thick electrolyte layers (or foils). In order to eliminate these disadvantages, the substrate concept with a flat geometry has been developed. This includes a porous substrate consisting of one of the two electrode materials which fulfills a support function and which, as a result, needs to be relatively thick in order to permit cells of reasonably large sizes to be manufactured. On this substrate a very thin electrolyte layer is deposited by a thin film manufacturing process and, in a next step, the second electrode layer is deposited on this electrolyte layer by one of the usual manufacturing processes. These three-layer cell units are assembled in series to a stack with the usual bi-polar plates [T. Iwata and H. Shundo, 2.sup.nd Symp. SOFC Japan, Tokyo, Dec. 15-16, 1993, Extended Abstract No. 101, p. 1-4].
The state of the art has the following disadvantage: If natural gas is fed into the anode space, the methane vapor reformation reaction occurs directly at the metal/YSZ-Cermet since the metallic phase (for example NI) acts in accordance with the state-of-the-art as a catalyst with respect to the methane-vapor reformation reaction. This reaction is strongly endothermic (.increment.+227.5 kg J/mol at 1000.degree. C.) arid accordingly removes heat from its environment. In addition, the reaction rate of this reaction is very high as compared to the following electrochemical reactions (at 900.degree. C. factor 40). As a result, the reformation reaction is completed already within a distance of 10 mm from the point where the methane vapor enters the anode chamber. The heat required within such a short distance cannot be supplied by the much slower electrochemical reactions, so that the temperature drops. For this reason, the stack variations referred to above require a pre-reformer in the periphery where some o the methane-vapor reformation reactions occur in order to avoid large temperature gradients in the stack, that is, in order to achieve a more uniform temperature distribution.
It is the object of the present invention to provide an anode substrate, which has disposed thereon a bi-polar plate for a high temperature fuel cell with which a uniform temperature distribution can be achieved in a simple manner.